Field of the Invention
The invention relates to a process for producing amplified negative resist structures.
In the fabrication of microchips, semiconductor substrates are structured (or patterned) using thin layers of photoresists. The chemical nature of the photoresists can be selectively altered by exposing them using a photomask or by direct irradiation, with an electron beam, for example. Following a developing step, in which either the exposed or the unexposed areas of the photoresist are removed, a structured photoresist is obtained which is used as a mask for etching the semiconductor substrate, for example. In the case of dry etching, the etching operation is usually performed with a fluorine or oxygen plasma. In order to selectively etch only the bare areas of the substrate, therefore, the mask-forming resist structure must possess sufficient resistance to the plasma that is used. When using an etching plasma containing oxygen, the photoresist therefore usually includes groups containing silicon. In the course of the etching operation, these groups are converted into silicon dioxide, which forms an etch-stable protective layer on the photoresist. The silicon atoms either may already be present in the photoresist polymer or may be introduced into the polymer subsequently, following the development of the resist structure, in a consolidation reaction. For this purpose, reactive groups are provided in the polymer. Such reactive groups include acid anhydride groups, carboxyl groups, or acidic phenolic hydroxyl groups, to which the amplifying agent, which carries a corresponding reactive group, an amino group for example, can be chemically attached.
In order to be able to realize low exposure doses and short exposure times when exposing the photoresist, photoresists known as chemically amplified resists (CARs) have been developed. In this case, the photoresist includes a photosensitive compound that on exposure, liberates a catalyst. In a subsequent amplifying step, the catalyst is able to bring about a chemical reaction that gives rise to a marked change in the chemical nature of the photoresist. With a single quantum of light, which liberates one catalyst molecule, it is therefore possible to bring about a multiplicity of chemical reactions and so to achieve a marked differentiation between the exposed and unexposed areas of the photoresist. The catalyst used is usually a strong acid, which is liberated by a photoacid generator, an onium compound for example. The polymer contains acid-labile groups, such as tertiary butyl groups, which are eliminated under the action of the strong acid liberated. The elimination of the acid-labile group is generally accompanied by the liberation of an acidic group: for example, a carboxyl group or an acidic phenolic hydroxyl group. This brings about a marked change in the polarity of the polymer, i.e., in its solubility in polar solvents. The polymer originally used in the photoresist, carrying acid-labile groups, is soluble in apolar solvents or solvent mixtures having a low polarity, such as alkanes, but also in alcohols, ketones or esters, whereas the polymer following the elimination of the acid-labile groups is soluble in polar solvents, generally water or basic, aqueous-organic developer solutions.
In connection with the production of resist structures, a range of processes have already been developed, which can be divided into two groups according to principles of operation.
In the case of positive photoresists, the exposed areas of the photoresist are detached in the developing step and in the structured photoresist, for example, form trenches, whereas the unexposed areas remain on the substrate and form, so to speak, the lines of the photoresist structure.
For producing positive photoresist structures, the procedure described above can be followed. As a result of the exposure, a chemical reaction is initiated within the photoresist, by means of which the photoresist polymer becomes soluble in alkaline developer solutions: for example, a 2.38% strength solution of tetramethylammonium hydroxide in water. On development, then, a corresponding positively structured photoresist is obtained.
In the case of negative resists, in contrast to the positive-working resists, the exposed portion of the resist remains on the substrate whereas the unexposed portion is removed by the developer solution. When working with chemically amplified negative resists, exposure initially likewise liberates a catalyst, usually a strong acid. The catalyst brings about, for instance, a crosslinking reaction in the photoresist, as a result of which the solubility of the polymer in the developer medium is reduced. As a result of the crosslinking, the exposed area becomes insoluble, whereas the unexposed area can be removed in appropriate developers. Developers used are generally aqueous solutions, so that the polymer usually has polar groups in the unexposed state.
For a modification of the developing step, a positive photoresist can also be used to produce a negative resist structure. A process of this kind is described, for example, in U.S. Pat. No. 4,491,628. There, a layer of a positive photoresist that is applied to a substrate is first of all exposed as described above, and an acid is liberated from a photoacid generator. In the subsequent amplifying step, the acid-labile groups in the exposed areas are eliminated by heating, so that the polymer is then in a polar form. In contradistinction to the positive developing process described above, an exposure is then carried out with an apolar solvent instead of with a polar aqueous developer. As a result, only the unexposed areas of the substrate, in which the polymer has retained its original apolar form, are detached. Since the polar fractions of the resist, in which polar groups—carboxylic acid groups, for example—have been produced by the exposure, are insoluble in apolar solvents, they remain as lines on the substrate.
Another negative photoresist includes not only a photobase, but also a thermoacid. A resist of this kind is described, for example, in Published PCT Patent Application PCT/DE00/04237. On exposure of the photoresist, a base is liberated in the exposed areas. In a subsequent amplifying step, an acid is liberated from the thermoacid generator by heating. In the exposed areas, the acid is neutralized by the base liberated beforehand and is therefore no longer available as a catalyst. In the unexposed areas, the acid catalyzes the elimination of the acid-labile groups from the polymer. Accordingly, in the unexposed areas, the polymer is converted from its apolar form into a polar form. In the subsequent developing step, therefore, the unexposed areas can be selectively detached from the substrate using an aqueous-alkaline developer, while the exposed areas remain, so to speak, as lines on the substrate.
As already mentioned, the resist structure must possess sufficient etch resistance when the substrate is etched. For this purpose, for instance, the lines of the resist structure must have a sufficient layer thickness. This is a particular problem in the case of resists for the 157 nm and the 13 nm technology, since at these wavelengths the photoresists known to date exhibit high absorption. Accordingly, only very thin polymer films can be used, in order to ensure that the radiation used for exposure is able to penetrate even into the deep areas of the resist in sufficient intensity, in order to be able to liberate sufficient quantities of catalyst. If insufficient quantities of catalyst are liberated in the lower layers of the photoresist, elimination of the acid-labile groups is incomplete, or in a worst case scenario, does not take place at all. A consequence of this is that following development, residues of the polymer remain in the trenches, forming what are known as resist feet. Because of its low layer thickness, the resistance of the structured photoresist to an etching plasma is insufficient, which is why its etch resistance must be increased. For this purpose, following development, the structured resist is chemically amplified. Where the resist structures have a sufficient layer thickness, it is also possible, in addition to an increase in layer thickness, to bring about a narrowing of the trenches, perpendicularly to the substrate surface, by laterally growing layers on the sidewalls of the trenches of the structured resist. As a result it is possible to achieve an improvement in resolution: that is, for example, the reproduction of narrower conductor tracks. A process of this kind is described, for example, in Issued European Patent Application EP 0 395 917 B1. In order to amplify the resist structure, the amplifying agent, in a solution in a suitable solvent or else from the gas phase, can be applied to the structured resist. The incorporation of silicon-containing amplifying agents into the polymer is generally referred to as silylation.
In a fluorine plasma, volatile silicon tetrafluoride is formed from the silicon that is present in the resist. In this case, amplifying the structured resist by means of silicon atoms makes no sense. In order to raise the resistance of the resist toward a fluorine plasma, therefore, the structured resist is amplified using aromatic amplifying agents.
In order to transfer structures produced with very short wavelength exposing radiation into a substrate, a resist system consisting of two layers has been used to date. The top layer of the resist system is comparatively thin and photostructurable. Following exposure, contrasting, and developing, the structured resist is amplified with a silicon-containing amplifying agent and the structure is transferred into the bottom layer of the resist system using an oxygen plasma. The bottom layer is composed, for example, of a resist which, although having a low etch resistance toward an oxygen plasma, possesses a high etch resistance toward a fluorine plasma. A resist of this kind includes polymers having a high aromatic fraction. An example is an etch-resistant novolac, a cresol resin. After the structure of the top, silylated resist layer has been transferred into the bottom resist layer with an oxygen plasma, the plasma is changed and the structure is transferred into the substrate using a fluorine plasma. The substrate in this case is composed, for example, of silicon, silicon nitride or silicon dioxide, so that the material of the substrate can be ablated by the conversion of the silicon-containing substrate into volatile silicon tetrafluoride. Because of the two-layer resist, the process is relatively expensive and technically complicated by the change of plasma system.
The existing processes for producing amplified resist structures involve a multiplicity of worksteps and are therefore very complicated to carry out. Every workstep also increases the error rate in the fabrication of microchips, meaning that a correspondingly high rejection rate must be tolerated. This is also a problem because of the fact that nondestructive testing is not possible at every step in microchip fabrication. Generally, error testing of this kind is possible only after several production steps, since it is only then that the electrical connections necessary for testing are present in the microchip. In some circumstances, therefore, several weeks may pass between a production step and error testing. Accordingly, every production step must exhibit an extremely low error rate.
Chemical consolidation requires corresponding reactive “anchor” groups in the polymer, to which the amplifying agent can be attached. Preparation of these polymers necessitates processes that are likewise complex, since, for example, they must be carried out in the absence of moisture in order to prevent premature hydrolysis of the reactive anchor groups.